In the course of processing a photographic element containing an imagewise exposed silver halide emulsion layer, reduced silver can be formed either as a direct or inverse function of exposure. At the same time, at least a low level of reduced silver formation also occurs independently of imagewise exposure. The term "fog" is herein employed to indicate the density of the processed photographic element attributable to the latter, usually measured in minimum density areas. In color photography, fog is typically observed as image dye density rather than directly as silver density.
A common disadvantage of fog-inhibiting agents is that they concurrently inhibit fog and reduce photographic speed to an increasing degree as they are increased in concentration in a silver halide emulsion. Thus, the choice of a particular fog inhibiting agent for use in a silver halide emulsion is based on both fog and photographic speed considerations, hereinafter referred to as speed/fog relationships.
Over the years a variety of differing materials have been introduced into silver halide emulsions to inhibit the formation of fog. Research Disclosure, Vol. 176, December 1978, Item 17643, Section VI, lists the more commonly employed fog inhibiting agents. Research Disclosure is published by Kenneth Mason Publications, Ltd., The Old Harbourmaster's, 8 North Street, Emsworth, Hampshire P010 7DD, England.
It has been generally recognized in the art that a particularly effective class of fog-inhibiting agents is comprised of quaternized thiazolium and selenazolium salts. By contrast quaternized oxazolium salts are not effective fog-inhibiting agents.
Brooker et al U.S. Pat. No. 2,131,038 discloses thiazolium salts, including a simple cyanine dye, to be useful fog-inhibiting agents. Mifune et al U.S. Pat. No. 4,237,214 discloses benzothiazolum salts having quaternizing substituents that can contain a carbamoyl or sulfamoyl group.
Gunther et al U.S. Ser. No. 660,155, filed Oct. 14, 1984, titled PHOTOGRAPHICALLY USEFUL CHALCOGENAZOLES, CHALCOGENAZOLINES, AND CHALCOGENAZOLINIUM AND CHALCOGENAZOLIUM SALTS, commonly assigned, discloses the preparation of aromatic tellurazolium salts and their utility as antifoggants.
In addition to the foregoing patents relating to fog-inhibiting agents, the following patents are of interest by reason of compound fragment similarities:
Nys et al U.S. Pat. No. 3,282,933 discloses polymethine dyes having a quaternizing substituent containing a divalent --CO--HN--SO.sub.2 -- group.
Herz U.S. Pat. Nos. 4,374,196 and 4,423,140 teach hydrolyzed quaternized chalcogenazolium salts to be useful latent image stabilizers in silver halide emulsions where the nitrogen atom contained in the ring prior to hydrolysis is substituted with an allyl group which may in turn be optionally substituted with an alkyl, alkoxy, carboxy, alkoxycarbonyl, or aminocarbonyl group.